Primary fibrosarcoma and malignant fibrous histiocytoma of bone--a comparative ultrastructural study: evidence of a spectrum of fibroblastic differentiation.

As primary bone fibrosarcoma (FS) and malignant fibrous histiocytoma (MFH) have similar clinical, radiographic, or survival manifestations, ultrastructural and immunohistochemical studies were undertaken to determine the differentiation pathways of constituent malignant cells. Twelve cases of primary intraosseous FS and MFH were selected for this ultrastructural comparative study and were analyzed for fibroblastic or modified fibroblastic differentiation. There were 4 FS cases and 8 MFH cases, of which 5 were storiform-pleomorphic, 2 were giant cell, and 1 was myxoid type. All FS consisted of spindle fibroblasts with a prominent rough endoplasmic reticulum and Golgi apparatus, variable amounts of vimentin intermediate filaments, and extracellular collagen fibrils. The MFH were composed of a mixture of spindle and pleomorphic fibroblasts (8/8), histiofibroblasts (4/8), and myofibroblasts (3/8). Variable numbers of undifferentiated cells were found in both tumors. In conclusion, fibroblastic differentiation and collagen production was noted in all cases. The often pleomorphic histiofibroblasts present in some MFH cases most likely represent "modified fibroblasts," similar to myofibroblasts. These findings support the hypothesis that the fibroblast and its variants are the predominant cell types found in these tumors, suggesting that the diagnostic entity MFH should be classified as a pleomorphic fibrosarcoma.